1. Field of the Invention
This invention relates to a process for producing a ferromagnetic liquid, and more specifically, to a process for producing a ferromagnetic liquid comprising fine particles of a ferromagnetic material and a surface-active liquid.
2. Description of the Prior Art
Magnetic liquids are liquid state magnets, and their utility in such fields as vacuum rotating shaft seals, ink jet printers and gravity concentration has already been discovered or is being considered. They are also expected to have extensive application to electric wave absorbers, thermal energy converting materials, magnetooptical elements, etc.
Magnetite (Fe.sub.3 O.sub.4) colloid has been used mainly as such a magnetic liquid. It is produced by (1) a wet pulverizing method which comprises pulverizing a block of magnetite in a colloidal dispersion medium composed of a mixture of water and a surface-active agent in a ball mill for an extended period of time (5 to 20 weeks), and separating large particles to prepare a magnetic liquid; or (2) a wet precipitation method which comprises adding an alkali to a mixed aqueous solution of a ferrous salt and a ferric salt to coprecipitate fine particles of magnetite and thereafter peptizing them to prepare a magnetic liquid. The wet pulverization method (1) is described, for example, in U. S. Patents 3,215,572 to S. S. Papell, 3,917,538 to R. E. Rosensweig 3,764,540 to S. E. Khalafalla and G. W. Rimers, and R. Kaiser and G. Miskolczy, J. Appl. Phys. 41 (1970), 1064, and the wet precipitation method is described, for example, in W. C. Elmore, Phys. Rev. 54 (1938), 309, E. E. Bibik, Kolloidnyi Zh. 35 (1973), 1141, and J. Shimoiizaka, K. Nakatsuka, R. Chubachi and Y. Sato, Nippon Kogyo Kaishi 93 (1977), 83.
Since the wet pulverization method requires a long period of pulverization and a step of separating coarse particles after pulverization, it has a very low production efficiency and the efficiency of utilizing the raw material is poor owing to the separation of coarse particles. Furthermore, because of the theory of this method, the particle diameters of the pulverized particles are distributed over a broad range, and therefore, it is difficult to control the properties of the resulting magnetic liquid and their quality. Another defect is that only soft and brittle materials such as magnetite can be applied to this method as a magnetic material, and the method is difficult to apply to tough and ductile materials such as metals or alloys.
On the other hand, the wet precipitation method utilizes the coprecipitation reaction of iron salts, and is therefore limited to ferromagnetic oxides such as magnetite. It is difficult to apply to a wide range of ferromagnetic materials. Furthermore, the particle diameters of the fine particles obtained by this method are within the range of 100 to 200 .ANG. and uniform within this range, but finer particles are difficult to obtain by this method.
The most important parameter which characterizes the performance of a magnetic liquid is the magnitude of its magnetization. A magnetic liquid obtained by using a magnetite colloid is limited in its performance because the magnetization of magnetite itself is low. The fundamental solution to this problem is to use a colloid composed of fine particles of a ferromagnetic material, for example ferromagnetic metals such as iron and cobalt having high magnetization, ferromagnetic alloys such as an iron-cobalt alloy or an iron-nickel alloy, and ferromagnetic compounds such as Heuster Alloy and Laves phase compounds. Since the ferromagnetic liquids are composed of ferromagnetic particles with a high saturation magnetization, they are liable to agglomerate and lose stability if their particle diameter exceeds 100 .ANG.. Hence, they should have a particle diameter of not more than 100 .ANG..
As an example of the preparation of a colloid of a ferromagnetic material, J. R. Thomas reported in J. Appl. Phys. 37 (1966), 2914 a method of producing a magnetic liquid composed of a cobalt colloid which comprises thermally decomposing cobalt carbonyl [Co.sub.2 (CO).sub.8 ] in toluene. The cobalt colloidal particles obtained by this method have a particle diameter of about 200 .ANG. and suffer from the defect of being liable to agglomerate in a dense colloid solution.